Hope you’re able to control your anger before approaching to him, as we hope there isn't any violence to be seen. Honest opinion, no point finding the guy out to talk. The problem is your wife having affair with another person. Talk to your wife and calmly hear the reasons for her actions. If there’s a need for forgiveness, remember that life becomes easier when you learn to accept an apology you never got.

Didn't know it was the fourth time. I’m sure you have been tolerating all these while with her irresponsible behaviors. For the sake of your son, do what you think is right. If you want to get custody of your son, then you must be ready to provide him the love of both mother and father, until you are able to find a wife that love you and your son wholly. Or you can give custody to your wife to take care of the kid, and support them financially. The best is try talking with your friends and families; as they are able to provide you a better direction.